In LTE (Long Term Evolution) of the 3GPP (3rd Generation Partnership Project), investigation of a SON (Self Organizing Network) is carried out from the viewpoint of reducing operation cost (OPEX: Operation Expenditure).
A SON has functions such as self-configuration, self-optimization, and self-healing, and technology is being investigated in related fields in order to realize different respective objects thereof (Non Patent Literatures 1, 2 and 3).
In SON technology, coverage (cell size), capacity, interference, cell topology, frequency allocation, bandwidth, and the like are automatically changed, set, and optimized.
Examples of radio parameters for implementing these include:
transmission power of radio base station (which may be abbreviated to eNodeB or eNB: evolved Node B or enhanced Node B, or simply “base station”),
antenna tilt angle of radio base station,
neighboring cell information, and
handover parameter,
wherein in order to realize the objects as described above, one or more of these radio parameters is changed as appropriate at the radio base station or operation administration and maintenance server (OAM server or SON server).
First, the following describes the related technology regarding coverage optimization. FIG. 10 is a diagram illustrating the related technology of optimization of coverage between macro cells. In FIG. 10, macro cell radio base stations (eNB1, eNB2) are connected to an operation administration and maintenance server (OAM/SON server). A certain radio terminal (known as UE: User Equipment) resides in a macro cell (Cell1), and performs communication with the radio base station 1 (eNB1).
The eNB1 gives an instruction to the UE to measure received quality of a downlink reference signal (which is a known signal and also termed as “pilot signal”) from the eNB1, and received quality of a downlink reference signal from a radio base station eNB (for example, radio base station 2 (eNB2)) of a neighbouring cell, and to make a report in a case where a predetermined preset condition is satisfied. Based on the instruction from the eNB1, the UE measures received quality of the downlink reference signal, and makes a measurement report of a measurement result of the received quality of the downlink reference signal, to the eNB1. The eNB1 reports the measurement result reported from the UE to the OAM/SON server. In the example of FIG. 10, the eNB2 also similarly reports a measurement result from a UE under its control to the OAM/SON server. The OAM/SON server, on receipt of the reports of the measurement results from the eNB1 and eNB2, instructs the eNB1 and eNB2 to adjust radio parameters and the like in order to optimize radio coverage (adjustment for optimization). For example, in the eNB1 and eNB2, coverage optimization (adjustment of transmission power, antenna tilt angle, or the like) for its own cell is executed in accordance with an instruction, from the OAM/SON server, to adjust a radio parameter or the like in order to optimize radio coverage.
FIG. 11 cites a drawing in Non Patent Literature 4. The flowing describes an example of coverage optimization, using an example of WCDMA (Wideband Code Division Multiple Access) of FIG. 11. Here, received quality measured by the radio terminal (UE) is the ratio of desired signal power to interference signal power (Ec/Io). Eo/Io is measured at the radio terminal (UE) and a measurement result is reported to the radio base station. Statistical information of measurement results is acquired at a base station or OAM. In the base station or the OAM, for example, the following calculation is performed:R=(ratio of terminals where Ec/Io exceeds a threshold).In a case where the calculated R is larger than a target, the base station or the OAM performs a unit step down of the transmission power of the radio base station. In a case where the calculated R is smaller than the target, the base station or the OAM performs a unit step up of the transmission power of the radio base station. It is to be noted that in LTE, use of RSRQ (reference signal received quality) may be considered instead of Ec/Io.